Warning device for distance between cars

ABSTRACT

A warning device for a distance between cars, for measuring a distance between a car and an object in front of the car, judging the possibility of danger on the basis of distance data thus obtained, and generating an alarm in the case where it is judged to be dangerous. The warning device provides a first alarm (a rear-end collision alarm) generated in a condition of alert which necessitates deceleration or braking, and a second alarm (an alarm for a distance between cars) generated in a condition of alert which does not necessitate deceleration nor braking, the first and second alarms, respectively, being judged by separate formulae for determining. Accordingly, it is possible to positively generate an alarm when deceleration or braking is necessitated. Further, the warning device discriminates a kind of an object being measured (a moving body, obstruction, guardrail or the like) to judge danger according to the kind of the discriminated object. Accordingly, it is possible to improve reliability of an alarm and, in particular, to reduce incorrect alarms such as by a guardrail.

TECHNICAL FIELD

This invention relates to a highly reliable car-to-car distance alarmdevice for use in an automobile, endowed with two kinds of alarm, i.e. acar-to-car alarm and a rear-end collision alarm, adapted to emit thesealarms by using two kinds of formula of decision and discriminate thekind of object for measurement, judge the danger ascribable to thediscriminated kind of object, and thereby allow infallible issuance ofan alarm in a state necessitating deceleration or manipulation ofbrakes, and furthermore enabled to reduce incorrect alarms by aguardrail etc.

BACKGROUND ART

Recently, as one approach to the promotion of convenience of anautomobile, a device called "car-to-car distance alarm device for anautomobile" which, in observance of the concept of active safetydirected to prevention of an accident, is adapted to measure thedistance from the user's own automobile to a preceding automobile, judgethe danger of collision of the two automobiles and, on detecting thedanger, issue an alarm to urge the driver's attention has been developedand adopted for practical use.

In the conventional car-to-car distance alarm devices for use in anautomobile, the judgement of the presence or absence of a dangeroussituation is generally accomplished by emitting one beam or three beamsof laser light thereby measuring the distance to an object formeasurement and comparing the measured distance with the safetycar-to-car distance in accordance with one formula of decision. Some ofthese devices, however, are adapted such that the user is allowed toadjust freely the timing of issuing an alarm by adequately varying thesafety car-to-car distance used in the formula of decision (refer toJP-A-58-10,198, for example).

In the car-to-car distance alarm devices for an automobile which areconstructed as described above, however, one formula of decision isrelied on to judge the question whether or not the alarm is to beissued. Generally speaking, this fact is suspected to bring aboutvarious disadvantages when the user has set the issuance of an alarmdifficult. When the user's automobile is involved in traffic congestion,for example, the user is apt to set difficult the issuance of an alarmwith a view to avoiding an unwanted addition to the noise of alarm. Thisfact implies the possibility that the issuance of an alarm will failnotwithstanding the situation generally threatens high danger ofrear-end collision. Even some of the devices of the type which do notallow the user to adjust the timing of issuing an alarm are adapted torefrain from issuing an alarm when the user's automobile is running at aspeed not more than a fixed level for the purpose of avoiding frequentissuance of an alarm during a course of traffic congestion. They havethe possibility of posing the same problem.

The conventional car-to-car distance alarm devices for an automobileemit the laser light in one beam or three beams for the measurement ofthe distance. They are, therefore, incapable of discriminating betweensuch items as guardrails, road signs, and other facilities attendant onroads, slopes of mountains, and walls of buildings which obstruct thetravel of the user's own automobile and such items as vehicles runningahead and obstacles on roads which can obstruct the travel of the user'sown automobile and often tend to issue an erroneous alarm such as by aguardrail.

It is, therefore, an object of this invention to provide a highlyreliable car-to-car distance alarm device for an automobile, which iscapable of infallibly issuing an alarm in a state necessitatingdeceleration or manipulation of brakes, even in the case where the userhas set the issuance of an alarm difficult.

It is a further object of this invention to provide a highly reliablecar-to-car distance alarm device for an automobile, which can reduceincorrect alarms such as by a guardrail.

DISCLOSURE OF THE INVENTION

This invention is directed to a car-to-car distance alarm device for anautomobile, adapted to measure a distance between a user's ownautomobile and an object existing ahead of the own automobile, judge thepresence or absence of danger based on distance data thus obtained, andissue an alarm on the case where it is judged to be dangerous, whichdevice is provided with a first alarm to be issued when it is in such astate as urges alert and necessitates deceleration or application ofbrakes and a second alarm to be issued when it is in such a state asdeserves alert and yet falls short of necessitating deceleration orapplication of brakes and enabled to judge the question whether thefirst alarm is issued, the second alarm is issued, or neither of thealarms is issued in accordance with a first formula of decision formedof one or more formulas of decision concerning a safety car-to-cardistance and a second formula of decision concerning a difference ofspeed between the own automobile and the object. Owing to thisconstruction, the device is enabled to issue the alarm (first alarm)infallibly in a state necessitating deceleration or application ofbrakes.

This invention is also directed to the car-to-car distance alarm devicefor an automobile mentioned above, which is provided with distancemeasuring means for emitting a light beam in a prescribed plurality ofdirections and measuring a distance to an object in the plurality ofdirections, first discriminating means for comparing a change of thedistance measured in the first and the second measurement in the samedirection by the distance measuring means with a distance of travel ofthe own automobile and discriminating whether an object existing in thatdirection is a moving object or a fixed object, second discriminatingmeans for, when the first discriminating means has judged the presenceof not less than a prescribed number of fixed objects ahead of the ownautomobile, regressing distance data thereof to find the standarddeviation and comparing the obtained standard deviation with aprescribed value thereby discriminating whether or not the fixed objectsare exclusively nonobstacles incapable of obstructing the travel of theown automobile, danger deciding means for deciding the presence orabsence of danger by a prescribed formula of decision, depending on thekind of object discriminated by the first discriminating means or thesecond discriminating means, and alarm issuing means for issuing aprescribed alarm when the danger deciding means has judged the presenceof danger. The reliability of an alarm can be exalted and particularlyan incorrect alarm by a nonobstacle such as a guardrail can be reducedby discriminating the kind of object under measurement and executing thedecision of danger in conformity to the discriminated kind of object asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the construction of a car-to-cardistance alarm device for an automobile as a preferred embodiment ofthis invention.

FIG. 2 is a schematic diagram illustrating the construction of a displaypart 10 shown in FIG. 1.

FIG. 3 is an explanatory diagram concerning the contents of the mode tobe set by the user.

FIG. 4 is a diagram to aid in the description of the contents of acar-to-car distance alarm.

FIG. 5 is a main flow chart illustrating schematically the operation ofan operation processing part 9 shown in FIG. 1.

FIG. 6 is a flow chart of the distance measuring process shown in FIG.5.

FIG. 7 is a flow chart of the data processing shown in FIG. 5.

FIG. 8 is a flow chart of the processing of discrimination performed onan object for measurement shown in FIG. 5.

FIG. 9 is a flow chart of the processing of judgment of danger shown inFIG. 5.

FIG. 10 is a flow chart of the processing of judgment of dangerregarding a vehicle shown in FIG. 9.

FIG. 11 is a flow chart continuing to step S69 in the flow chart shownin FIG. 10.

FIG. 12 is a flow chart continuing to step S70 in the flow chart shownin FIG. 10.

FIG. 13 is a flow chart of the processing of judgment of dangerregarding a fixed object shown in FIG. 9.

FIG. 14 is a flow chart continuing to step S98 in the flow chart shownin FIG. 13.

FIG. 15 is a flow chart continuing to step S99 in the flow chart shownin FIG. 13.

FIG. 16 is a flow chart of the processing of alarm display shown in FIG.5.

FIG. 17 is a diagram to aid in the description of the processing ofdiscrimination performed on an object for measurement which is in thesituation of traveling along a curve in a road.

FIG. 18 is a diagram to aid in the description of the processing ofdiscrimination performed on an object for measurement which is in thesituation of traveling along a corner of a T road.

FIG. 19 is a diagram showing one example of the relation between avehicle speed V and a braking distance f(V).

FIG. 20 is a diagram showing one example of the relation between avehicle speed V and a car-to-car distance d during a safety stop.

FIG. 21 is a diagram showing one example of the relation between a speedV₂ of the user's own automobile and a reference value X(V₂) forcomparison.

BEST MODE FOR CARRYING OUT THE INVENTION

This invention will be described in detail below with reference to thedrawings annexed hereto.

FIG. 1 illustrates the construction of a car-to-car distance alarmdevice for an automobile as a preferred embodiment of this invention.

This car-to-car distance alarm device of this invention is provided witha light projecting part 1 for sequentially and intermittently emitting abeam of laser light having a prescribed wavelength (a near infraredlaser beam, for example) in a prescribed plurality of directions (sixdirections, for example). This light projecting part 1 preferablyincorporates therein a light source such as a semiconductor laser diodefor oscillating a near infrared laser beam and an emission plate forvarying the angle of emission of the near infrared laser beam. To thelight projecting part 1 is connected a pulse generating part 2 whichgenerates a pulse signal for causing intermittent oscillation of thenear infrared laser beam at a prescribed cycle in the light sourcementioned above. The emission plate incorporated in the light projectingpart 1 is driven by an emission plate driving motor 3 within aprescribed range of motion. The emission plate driving motor 3 is drivenby a motor driving part 4. The motor driving part 4 is composed of sucha power element as a power transistor, a signal converting part forpositioning the motor 3, etc. The reflected light which is produced whenthe near infrared laser beam emitted from the light projecting part 1impinges on an object for measurement is detected by a light receivingpart 5. To the light receiving part 5 is connected a time/voltageconverting part 6 which converts the time intervening between a point atwhich the light projecting part 1 emits the near infrared laser beam anda point at which the beam returns to the light receiving part 5 afterbeing reflected on the object under measurement into a voltage. Thistime/voltage converting part 6 is connected also to the pulse generatingpart 2 so as to establish synchronization with the timing of emission ofthe near infrared laser beam. A voltage analog signal (time data)emitted from the time/voltage converting part 6 is converted by an A/Dconverter 7 into a digital signal and then transmitted to an operationprocessing part 9 which will be mentioned later. To the A/D converter 7is also connected an own car's speed detecting part 8 for detecting aspeed of the user's own automobile. An analog signal (own car's speeddata) emitted from the own car's speed detecting part 8 is likewiseconverted by the A/D converter 7 into a digital signal and thentransmitted to the operation processing part 9. This operationprocessing part 9 is intended to compute the distance to an object undermeasurement based on the time data from the time/voltage converting part6 and decide the degree of danger based on the resultant distance data.The operation processing part 9 comprises microcomputers, for example,and incorporates such memories as ROM and RAM therein. The measurementof distance is performed in six directions as mentioned above. Byperforming this measurement of distance in the six directions up to tworepetitions a brief interval apart, the kind of object under measurementcan be discriminated as described specifically below. To the operationprocessing part 9 are severally connected the pulse generating part 2and the motor driving part 4 mentioned above. The pulse generating part2 and the motor driving part 4 are severally started/stopped by anoperation start/stop signal emanating from the operation processing part9 and are mutually synchronized. Further to the operation processingpart 9 is connected a display part 10 which fills the role of displayingthe outcomes of the arithmetic operations, namely the distance to theobject under measurement and the degree of danger and so forth, andissuing a relevant alarm. Incidentally, the distance measuring means iscomposed of the light projecting part 1, pulse generating part 2,emission plate driving motor 3, motor driving part 4, light receivingpart 5, time/voltage converting part 6, A/D converter 7, and operationprocessing part 9, the first and the second discriminating means anddanger deciding means are composed of the operation processing part 9,and the alarm issuing means is composed of the display part 10.

The present device is endowed with two kinds of car-to-car distancealarm, i.e. a rear-end collision alarm as the first alarm and acar-to-car alarm (particularly a car-to-car alarm in the narrow sense ofthe word, meaning "car-to-car danger alarm" which will be specificallydescribed herein below) as the second alarm, and they are decided byusing two kinds of formula of decision as will be described specificallyherein below. The term "car-to-car alarm" as used herein means the casein which the user's automobile approximates so closely to an object formeasurement ahead that the user must be alert and nevertheless can copewith the situation without deceleration or application of brakes and theterm "rear-end collision alarm" means the case in which the user'sautomobile approximates so closely to an object for measurement aheadthat the user must definitely resort to deceleration or application ofbrakes. Where the distance between other automobile ahead and the user'sautomobile is 10 m and the current traveling speed of the user'sautomobile is 50 km/h, for example, the car-to-car alarm is issued whenthe current traveling speed of other automobile is not less than 50 km/hbecause the user has no use for deceleration or application of brakes,whereas the rear-end collision alarm is issued when the currenttraveling speed of other automobile is less than 50 km/h because theuser must resort to deceleration or application of brakes.

The car-to-car distance alarm device for an automobile which isconstructed as described above can be mounted on various automobilessuch as passenger car, bus, truck, special vehicle, and two-wheelerwhich are chiefly used on roads. This device is constructed such that itcan detect an object falling in a zone about 100 m ahead and about 8.5 mwide. Specifically, the light projecting part 1 continuously varieswithin the prescribed range of motion the angle of the emission platedisposed therein by means of the emission plate driving motor 3 actuatedby the motor driving part 4 so as to emit the near infrared laser beamin six directions with the direction of this emission changed one degree(°) at a time and meanwhile oscillates the near infrared laser beam atthe prescribed cycle from the light source (semiconductor laser diode)in accordance with the pulse signal generated in the pulse generatingpart 2 which is synchronized with the variation of angle. In this case,therefore, the area of detection in the foreground is about 5 degrees(°) in terms of angle. The distance to an object for measurement aheadin a given direction can be measured by clocking the time whichintervenes between a point at which the near infrared laser beam isemitted and a point at which it is returned after being reflected on anobject for measurement falling in that direction. To be specific, thereflected light in each of the directions from the object formeasurement is detected by the light receiving part 5, the timeintervening between the point of emission from the light projecting part1 and the point of return to the light receiving part 5 after reflectionis measured in the form of the magnitude of voltage at the time/voltageconverting part 6, and the magnitude thus measured is transmitted viathe A/D converter 7 to the operation processing part 9. The operationprocessing part 9 computes the distance to the object for measurementbased on the data of time from the time/voltage converting part 6 andthe velocity of light. The measurement of distance in the six directionsis carried out by one scan in the manner described above.

FIG. 2 illustrates the construction of the display part 10 shown in FIG.1.

This display part 10 is roughly composed of an LED part 21 fordisplaying numerical values etc., an alarm lamp 22 for indicating thekind of alarm, a mode switch 23 for the user's own setting, an ON/OFFswitch 24 for the power source, and a speaker 25 for emitting a sound ofalarm. The LED part 21 comprises three 7-segment LED's 21a, 21b, and 21cand one decimal point LED 21d. The alarm lamp 22 comprises a green LED22a, an orange LED 22b, and a red LED 22c. The mode switch 23 for user'ssetting is composed of a mode selection switch 23a for selecting modes,an up (↑) switch 23b to be used in increasing the numerical valuedisplayed in the LED part 21 or selecting the ON state, and a down (↓)switch 23c to be used in decreasing the numerical value displayed in theLED part 21 or selecting the OFF state.

FIG. 3 explains the contents of the mode for the user's setting.

This invention contemplates judging the car-to-car alarm and therear-end collision alarm severally by using two kinds of formula ofdecision as described above. It also allows the user to adjust thetimings severally for emitting the car-to-car alarm and the rear-endcollision alarm at his own discretion (personal errors such as due tospeed of reaction and character). The adjustment of the timing forissuing an alarm is carried out, for example, by varying the so-calledfoot transfer time (the driver's retardation in reaction). The presentdevice allows even the ON/OFF of the sound of alarm, the alignment ofthe optical axis, the setting of the volume of the sound of alarm and soforth in addition to the setting of timing for a rear-endcollision/car-to-car alarm as described above. In this device, thecar-to-car alarm (in the broad sense of the word) comes in two kinds,i.e. a car-to-car danger alarm (a car-to-car alarm in the narrow senseof the word) and a car-to-car alert alarm. The car-to-car alert alarm isfurther divided into two steps, depending on the degree of danger ofcollision.

Specifically, the mode can be switched sequentially by giving a push ata time to the mode selecting switch 23a after the power source has beenturned on by the power source ON/OFF switch 24. The present device hasseven modes prepared for the user's setting. The number identifying theset mode is displayed on the 7-segment LED 21a. Mode 1, for example, isa mode for setting the timing for issuing the rear-end collision alarm.The user is allowed to set the foot transfer time T₂ for the rear-endcollision alarm freely by manipulating the up switch 23b and the downswitch 23c. The foot transfer time T₂ thus set for the rear-endcollision alarm is displayed on the 7-segment LED's 21b and 21c. Mode 2is a switch mode for turning ON/OFF the sound of alarm for the rear-endcollision alarm; the status ON is set by depressing the up switch 23band the status OFF the down switch 23c. The state of the ON/OFF settingis displayed on the 7-segment LED's 21b and 21c. Mode 3 is a mode forsetting the timing for issuing the car-to-car alarm; the foot transfertime T₁ for the car-to-car alarm can be freely set by the up switch 23band the down switch 23c. The foot transfer time T₁ so set for thecar-to-car alarm is displayed on the 7-segment LED's 21b and 21c. Mode 4is a switch mode for turning ON/OFF the sound of alarm of the car-to-caralarm in the narrow sense of the word (namely, the car-to-car dangeralarm); the status ON is set by depressing the up switch 23b and thestatus OFF by the down switch 23c. The state of the ON/OFF setting isdisplayed on the 7-segment LED's 21b and 21c. Mode 5 is a mode foraligning the optical axis. The alignment of the optical axis by thepresent device is attained with the aid of a test target. The alignmentthus effected can be announced as perfect when this device actuated withthe target placed at a stated distance (10 m, for example) ahead thereofexactly measures the distance to the target. Mode 6 is a mode forconfirming the current traveling speed of the user's own automobile. Thespeed of the own automobile detected by the own car's speed detectingpart 8 is displayed on the 7-segment LED's 21b and 21c. Mode 7 is a modefor setting the volume of sound of the alarm sound. The present deviceis allowed to set the volume of sound at the two steps, HI (large) andLO (small). The status of the level of the sound volume thus set isdisplayed on the 7-segment LED's 21b and 21c. In addition to the modesfor the user's setting described above, the present device is furnishedwith a standard mode for allowing the car-to-car distance measuredduring the operation of the device to be displayed in units of 1 m, forexample, on the 7-segment LED's 21a-21c. This standard mode is setimmediately after the power source has been turned on or it is set eachtime the mode selecting switch 23a is depressed once from the status ofMode 7 for the user's setting. Specifically, whenever the mode selectingswitch 23a is depressed once at a time, the mode is switchedsequentially in the order of standard mode→Mode 1→Mode 2→Mode 3→Mode4→Mode 5→Mode 6→Mode 7→standard mode, for example.

FIG. 4 explains the contents of the car-to-car distance alarm.

This embodiment contemplates broadly dividing the car-to-car distancealarm into a car-to-car alarm in the broad sense of the word which doesnot require such measure as application of brakes and a rear-endcollision alarm which requires such measure as application of brakes.Then, the car-to-car alarm in the broad sense of the word is dividedfirst by the degree of danger of collision into a car-to-car dangeralarm (a car-to-car alarm in the narrow sense of the word) and acar-to-car alert alarm and the car-to-car alert alarm is likewisedivided into two kinds. When the rear-end collision alarm is issued, thered LED 22c is flickered (display of rear-end collision alarm) and thespeaker 25 emits a sound of alarm where the sound of alarm of therear-end collision alarm has been set to the ON status. When thecar-to-car danger alarm is issued, the red LED 22c is lighted (displayof car-to-car danger alarm) and the speaker 25 emits a sound of alarmwhere the sound of alarm of the car-to-car alarm has been set to the ONstatus. When the car-to-car alert alarm is issued, the orange LED 22b islighted where the degree of danger is on the higher side in the twosteps or the green LED 22a is lighted where the degree of danger is onthe lower side (display of car-to-car alert alarm). No sound of alarm isemitted in the case of the car-to-car alert alarm. Incidentally, whenthe brakes happen to be applied during the life of the rear-endcollision alarm or the car-to-car danger alarm, the present deviceforbids the emission of a sound of alarm even where the sound of alarmhas been set to the ON status.

Now, the operation of this car-to-car distance alarm device will bedescribed below with reference to the flow charts of FIG. 5-FIG. 16.

FIG. 5 is a main flow chart schematically illustrating the operation ofthe operation processing part 9 shown in FIG. 1.

When the power source is turned on by the power source ON/OFF switch 24and the device is set to the standard mode, measurement of distance isfirst carried out at the step S1. In this measurement of distance, thenear infrared laser beam is emitted in six directions as shifted by onedegree (°) at a time to measure the distances of objects for measurementin the six directions. This measurement of distance in the sixdirections is performed up to two repetitions a brief interval apart.

Next, the data obtained at the step S1 is processed (step S2) and theresultant data is used for discriminating the kind of each of theobjects for measurement (step S3). This operation, in brief, consists incomparing the changes of distances found in the first and the secondmeasurement in the same directions with the distance of travel of theuser's own automobile thereby discriminating whether each of them is amoving object (an automobile such as a four-wheeler and two-wheeler inmotion etc.) or a fixed object (a vehicle at rest, obstacle, etc.) and,when the presence of the fixed object is confirmed, discriminatingwhether the fixed object is a small obstacle or large one, depending onwhether the fixed object is present in not less than three of the totalof six directions and, when it is found as the large obstacle,regressing the distance data and finding the standard deviation, andjudging whether those fixed objects are exclusively nonobstacles such asguardrails or includes other obstacles, depending on the magnitude ofthe found standard deviation.

Thereafter, the question as to whether or not the objects formeasurement found at the step S3 include those satisfying the conditionfor executing the decision on danger which will be describedspecifically herein below is judged at the step S4. When the answer isYES, the processing for judging danger is carried out (step S5) and theprocessing for displaying an alarm (step S6) is carried out inaccordance with the result consequently obtained. When the answer is NO,the flow of the process advances immediately to the step S7.Incidentally, the processing for judging danger at the step S5 isintended, as will be described afterward, to execute the judgment ofdanger, depending on the kind of an object for measurement and furtherdivide the alarm into a car-to-car alarm and a rear-end collision alarmon each occasion and emit these alarms by using two kinds of formula ofdecision.

The series of processing described above is repeated until completion ofthe series is indicated (step S7).

Now, the contents of the sub-routines mentioned above will be describedin detail below.

FIG. 6 is a flow chart of the process for measurement of distance shownin FIG. 5.

First in this processing for measurement of distance, the infrared laserbeam is emitted from the projector 1 in six directions to effect thefirst measurement of distance in six directions, the data of distance inthe six directions L₁₁, L₂₁, L₃₁, L₄₁, L₅₁, and L₆₁ consequentlyobtained are stored in the memory (RAM; similarly applicablehereinafter) (step S1), and the own car's speed v₁ during the firstmeasurement is detected by the own car's speed detecting part 8 and thenstored in the memory (step S12). Then, the second measurement ofdistance in the six directions is executed after an interval of a brieftime, the data of distance in the six directions L₁₂, L₂₂, L₃₂, L₄₂,L₅₂, and L₆₂ consequently obtained are stored in the memory (step S13),and the own car's speed v₂ during the second measurement is detected bythe own car's speed detecting part 8 and stored in the memory (stepS14).

FIG. 7 is a flow chart of the data processing shown in FIG. 5.

In this data processing, first the difference ΔL_(j) (j=1, 2, . . . , 6)between the distances obtained in the first and the second measurementin each of the directions is computed by the following formula,

    ΔL.sub.j =L.sub.j1 -L.sub.j2 (j=1, 2, . . . , 6)

and the result of the computation is stored in the memory (step S21).Then, the own car's average speed v during the first and the secondmeasurement is computed by the following formula (step S22),

    v=(v.sub.1 +v.sub.2)/2

and the difference of brief time Δt during the first and the secondmeasurement is found (step S23), the distance of own car's travel Δdduring the first and the second measurement is computed by the followingformula,

    Δd=v×Δt

and the result of this computation is stored in the memory (step S24).

FIG. 8 is a flow chart of the processing of discrimination of an objectfor measurement shown in FIG. 5.

In this processing for discrimination of an object for measurement,first the j value which is a parameter representing a direction is setat 1 (step S31), the change of distance between the first and the secondmeasurement in the direction j found at the step S21 is compared withthe distance of the own car's travel Δd found at the step S24, and thequestion as to whether or not the absolute value of the differencebetween the two distances (|ΔL_(j) -Δd|) is not more than a prescribedvalue (1 m, for example) is judged (step S32). When the judgment derivesYES as the answer, the object for measurement is judged to be a fixedobject incapable of motion (step S33) because the own car ought to havetraveled over substantially the change of the measured distances. Whenthe judgment derives NO as the answer, the object for measurement isjudged to be a moving object such as other vehicle (step S34). Since itis safe to conclude that most moving objects on a road are othervehicles, the following description presumes other vehicle as a movingobject.

When the step S33 or the step S34 is completed, the question whether ornot the j value is 6 is judged (step S35) and the flow of the processadvances to the step S37 when the answer is in the affirmative. When theanswer is in the negative, the j value is incremented by 1 (step S34)and the flow of process returns to the step S32. With respect to all thesix directions involved in the measurement, the question whether therelevant object for measurement is a fixed object or other vehicle isjudged. The result of this judgment is stored in the memory as relatedat least with the data of distance L₁₂ -L₆₂ obtained by the secondmeasurement.

When the processing of discrimination is completed with respect to eachof the six directions, the question whether or not a fixed object existsin the six directions is judged at the subsequent step S37. When thejudgment derives YES as the answer, the flow of process advances to thestep S38 to discriminate the kind of the fixed object more specifically.When the judgment derives NO as the answer, namely when nothing butother vehicle is detected, the process immediately returns and shifts tothe processing for judgment of danger.

When a fixed object is present, the question whether or not the fixedobject exists in not less than three, for example, of the total of sixdirections is judged first at the step S38. When the judgment derives NOas the answer, the fixed object is judged to be a small obstacle (stepS39) because this fixed object exists only in two directions at most inall the six directions. The process immediately returns and shifts tothe processing for judgment of danger.

When the judgment at the step S38 derives YES as the answer, the fixedobject is judged to be a large obstacle because it is present in notless than three of the total of six directions. Subsequently, thequestion as to whether this fixed object is exclusively a nonobstaclesuch as a guardrail which does not obstruct the own car's travel orincludes besides that an obstacle which obstructs the own car's travelis discriminated.

Specifically, the second data of distance L_(j2) which is judged to be afixed object is extracted from the memory (step S40), the extracted dataof distance L_(j2) is regressed to compute the standard deviation a(step S41) and the question whether or not the found standard deviationa is not more than a prescribed value (0.5, for example) is judged (stepS42). When this judgment derives YES as the answer, this fixed object isjudged to be an on obstacle such as a guardrail, road sign, slope of amountain, or wall of a building because the detected fixed object showsonly a sparing dispersion of position and is consequently judged to besmoothly bent or straight and the result of this judgment is stored inthe memory (step S43). When the judgment derives NO as the answer, thisfixed object is judged to include at least besides such as a guardrailan obstacle which obstructs the own car's travel because it shows toolarge a dispersion of position to justify a conclusion that it isexclusively such as a guardrail (step S45). In the former case, thecar-to-car distance d during a safety stop which is used for thecomputation of the safety car-to-car distance as will be morespecifically described hereinafter is set at 0.2 times the standardvalue (refer to the diagram of FIG. 20) (step S44) because this fixedobject is exclusively such as a guardrail and forms no obstacle to theown car's travel. When the object for measurement is then on obstaclesuch as a guardrail, therefore, the possible occurrence of an erroneousalarm due to such as a guardrail is markedly reduced because the safetycar-to-car distance is so short as to render the issuance of an alarmdifficult. In contrast, in the latter case, the process immediatelyreturns without any modification and shifts to the processing forjudgment of danger because the obstacle other than such as a guardrailis in need of ordinary judgment of danger. The correction of thecar-to-car distance d during a safety stop at the step S44 may becarried out when the safety car-to-car distances D, D₁, and D₂ arecomputed as described specifically herein below.

When an obstacle 33 exists on the foreground of a guardrail 32 ahead thecenter of an own automobile 32 in a curve of a road as illustrated inFIG. 17, for example, the standard deviation a computed by regressingthe data of distance L_(j2) (j=1, 2, . . . , 6) determined in all thesix directions turns out to be a large magnitude (absolute value)because of a dispersion of data due to the presence of the obstacle 33on the foreground. Thus, the obstacle 33 on the foreground of theguardrail 32 can be detected (step S45) by the judgment at the step S42.By the same token, when an obstacle 35 exists on the foreground of astraight guardrail 34 ahead the left side of an own automobile 31 in a Troad as illustrated in FIG. 18, the standard deviation a computed byregressing the data of distance L_(j2) (j=1, 2, . . . , 6) determined inthe six directions turns out to be a large magnitude (absolute value)because of the presence of the obstacle 35 on the foreground. Thus, theobstacle 35 on the foreground of the guardrail 34 can be detected (stepS45) by the judgment at the step S42. If the obstacle 33 or 35 does notexist in the situation of FIG. 17 or FIG. 18, the guardrail 32 or 34 isexclusively detected in front of the own car 31 (Step S43) by thejudgment of the step S42 because the standard deviation a obtained byregressing the data of distance L_(j2) (j=1, 2, . . . , 6) measured inthe six directions turns out to be a small magnitude.

When the processing for discrimination of the kind of an object formeasurement is completed as described above, the question whether thedetected objects for measurement include those which fulfill thecondition for the execution of judgment of danger is judged at the stepS4 shown in FIG. 5. This condition for the execution of judgment ofdanger forms the basis for judging whether or not the judgment of dangeris actually necessary and consists in preparatorily setting a prescribedarea with the directions (angles) and distances of the beams to beemitted and carrying out judgment of danger with respect exclusively toobjects which happen to exist within the area. This setting of the area,for example, may be implemented with emphasis laid on such objects asfall on the same lane that the own car travels on. Let Direction 1,Direction 2, Direction 3, Direction 4, Direction 5, and Direction 6reckoned sequentially from the left side onward stand for the directionsof scan of the beam, for example, and the areas will be so set that anarea of 25 m is assigned to each of the laterally outermost directions,i.e. Direction 1 and Direction 6, an area of 40 m to each of theinwardly adjoining directions, i.e. Direction 2 and Direction 5, and anarea of 100 m to each of the central directions, i.e. Direction 3 andDirection 4.

At the step S4, therefore, the question whether or not an object formeasurement answering any one of these expressions, L₁₂ <25 m, L₂₂ <40m, L₃₂ <100 m, L₄₂ <100 m, L₅₂ <40 m, and L₆₂ <25 m exists is judgedbased on the data of distance, ΔL_(j2) (j=1, 2, . . . , 6), obtained inthe second and consequently newer measurement in the total of twomeasurements to be involved. When this judgment derives YES as theanswer, all the objects for measurement that answer the relevantexpressions are picked up and stored in the memory. When the judgmentderives NO as the answer, the flow of process immediately advances tothe step S7 because all the objects for measurement fall at suchpositions as form no obstacle to the own car's travel and no judgment ofdanger is required.

FIG. 9 is a flow chart of the processing of judgment of danger shown inFIG. 5.

In this processing for judgment of danger, first the objects formeasurement answering the condition for execution of judgment of dangerfound at the step S4 are surveyed to extract the object having thesmallest distance L_(j2) of measurement (step S51) and the questionwhether or not the extracted object for measurement is other vehicle(step S52). When this judgment derives YES as the answer, the programfor judgment of danger with respect to a vehicle is executed (step S53).When the judgment derives NO as the answer, namely when the object formeasurement is a fixed object, the program for judgment of danger withrespect to a fixed object is carried out (step S54).

Though the process, as described above, contemplates executing theprocessing for judgment of danger exclusively with respect to the objectfor measurement approximating most closely to the own car among otherobjects for measurement which answer the condition for execution ofjudgment of danger, it does not need to be limited to this particularmode. Where other vehicles and fixed objects coexist, for example, it ispermissible to judge danger with respect severally to these items andthen display an alarm with respect to the item which has been judged tohave the highest degree of danger.

FIG. 10-FIG. 12 are flow charts of the processing for judging dangerwith respect to a vehicle shown in FIG. 9.

In this processing for judging danger with respect to a vehicle, firstthe speed V₁ of other vehicle ahead of the own car is computed by thefollowing formula (step S61).

    V.sub.1 =V.sub.2 +ΔL.sub.j /Δt

wherein

V₂ : own car's speed (the own car's speed v₂ during the secondmeasurement detected at the step S14, for example, may be used)

ΔL_(j) : difference of distances found in the first and the secondmeasurement in the direction j of the relevant other vehicle (the valuefound at the step S21 may be used)

Δt: difference of time between the first and the second measurement (thevalue found at the step S23 may be used)

Then, the question whether or not the weather is rainy (step S62). Whenthe weather is found to be rainy, the braking distance f₁ (V₁) of othervehicle and the braking distance f₂ (V₂) of the own car are severallycorrected, for example, to 1.5 times the standard value (FIG. 19 refers)(step S63) because the road has a slippery surface. As a result, thisdevice is adapted to the actual situation of the road. The term "brakingdistance f(V)" of a vehicle means the distance over which the vehicletravels between the time the brakes are applied and the time the vehicleis ultimately stopped. This distance varies with the speed V of vehicle.The magnitude of the braking distance f(V) is properly set in advance byusing as a parameter thereof the speed V of vehicle determinedempirically with respect to the standard situation which excludes thefactor of rainy weather. FIG. 19 depicts one example of the set ofbraking distances. The data of the braking distance f(V) which is setfor a particular automobile furnishes more accurate information. Wherethe weather is not rainy, the flow of process immediately advances tothe step S64 because the braking distance f(V) does not need to becorrected.

At the next step S64, the distance Lc to other car is set. Specifically,the distance L_(j2) to the relevant other car found in the secondmeasurement in the direction J is used as the value of the distance Lc.

Thereafter, the question whether or not the own car's deceleration isnot less than 10% per second is judged (step S65). This judgment iscarried out in accordance with the following formula, for example.

    (v.sub.1 -v.sub.2)/Δt≧0.1

When the own car's deceleration is less than 10% per second, the devicejudges that the driver has not yet noticed the presence of other car andhas not applied brakes and computes the safety car-to-car distance D bytaking into account the free running distance f₃ (T) due to the time ofdelay of the driver's reaction (foot transfer time) T. In thisinvention, since the car-to-car alarm and the rear-end collision alarmare judged as discrminated, the safety car-to-car distance D₁ whichtakes into account the foot transfer time T₁ for car-to-car alarm andthe safety car-to-car distance D₂ which takes into account the foottransfer time T₂ for rear-end collision alarm are computed separately ofeach other. Incidentally, the free running distance f₃ (T) is computedin accordance with the following formula.

    f.sub.3 (T)=V.sub.2 ×T

Specifically, the foot transfer time T₁ for car-to-car alarm which isset by the user is read out of the memory (step S66), the safetycar-to-car distance D₁ taking into account the foot transfer time T₁ forcar-to-car alarm is computed in accordance with the following formula(step S67),

    D.sub.1 =f.sub.2 (V.sub.1)+f.sub.3 (T.sub.1)-f.sub.1 (V.sub.1)-d

and the foot transfer time T₂ for rear-end collision alarm similarly setby the user is read out of the memory (step S68), the safety car-to-cardistance D₂ taking into account the foot transfer time T₂ for rear-endcollision alarm is computed in accordance with the following formula(step S69), and the flow of process is advanced to the next step S71.

    D.sub.2 =f.sub.2 (V.sub.2)+f.sub.3 (T.sub.2)-f.sub.1 (V.sub.1)-d

wherein

f₁ (V₁): braking distance of other vehicle

f₂ (V₂): own car's braking distance

f₃ (T₁): free running distance due to foot transfer time T₁

f₃ (T₂): free running distance due to foot transfer time T₂

d: car-to-car distance during safety stop

Incidentally, the car-to-car distance d during a safety stop is acar-to-car distance which exists when the own car is stopped safely andit varies with the own car's speed V₂. This magnitude is properly set inadvance by using as a parameter thereof the empirically determinedvehicle speed V. FIG. 20 depicts one example thereof.

When the own car's deceleration is found to be not less than 10% persecond by the judgment of the step S65, since the own car is judged tobe already in the process of deceleration initiated such as by theapplication of brakes and consequently to be not in need of taking intoaccount the free ruuning distance f₃ (T), the safety car-to-car distanceD is computed in accordance with the following formula (step S70) andthe flow of process is advanced to the next step S80.

    D=f.sub.2 (V.sub.2)-f.sub.1 (V.sub.1)-d

When the own car's deceleration is less than 10% and the setting of twosafety car-to-car distances D₁ and D₂ has been completed, the questionwhether or not the distance Lc to other car set at the step S64 is notless than the safety car-to-car distance D₁ determined at the step S67,namely the question whether or not the following formula (hereinafterreferred to "first formula A of decision") is fulfilled, is judged atthe next step S71.

    Lc-D.sub.1 ≧0

When this judgment derives NO as the answer (route P), the flow ofprocess immediately advances to the step S73. When the judgement derivesYES as the answer, further, similarly the question whether or not thedistance Lc to another car is not less than the safety car-to-cardistance D₂ found at the step S69, namely whether or not the followingformula (hereinafter referred to as "first formula B of decision") issatisfied, is judged (step S72).

    Lc-D.sub.2 ≧0

When this judgment derives YES as the answer, the car-to-car alert alarmis selected (step S77) because the degree of danger is not very highwithout reference to the choice between the foot transfer times T₁ andT₂ for the sake of consideration. The car-to-car alert alarm comes intwo steps, i.e. a green color and an orange color, as described above(FIG. 4 refers). Further, as regards the choice between these two stepsof alarm, the time allowed for applying the brakes is computed backwardfrom the difference (Lc-D₁) between the distance Lc to other car and thesafety car-to-car distance D₁ and the car-to-car alert alarm in thegreen color is selected when the found time allowance is not less than(T₁ +1.5) seconds or the car-to-car alert alarm in the orange color isselected when the time allowance is not less than (T₁ +0.5) seconds andless than (T₁ +1.5) seconds. In contrast, the judgment derives NO as theanswer (route Q), the flow of process advances to the step S73.

At the step S73, the difference of speed S (=V₂ -V₁) between the own carand the other car is computed. At the next step S74, the foot transfertime T₂ for rear-end collision alarm is read out of the memory and thereference value X(T₂) for comparison is found. The reference value X(T₂)is a set value which varies with the foot transfer time T₂. In view ofthe undeniable possibility of collision, it is properly set in advanceempirically. FIG. 21 depicts one example thereof.

Then, at the subsequent step S75, the question whether the difference ofspeed S found at the step S73 is greater than the reference value X(T₂)found at the step S74, namely whether the following formula (hereinafterreferred to as the "second formula of decision") is fulfilled, isjudged.

    S>X(T.sub.2)

When this judgment derives YES as the answer, the rear-end collisionalarm is selected (step S79).

In contrast, when the judgment derives NO as the answer, the questionwhether or not the route reaching the judgment of step S75 is P, namelythe question whether the judgment at the step S71 has derived NO or thejudgment at the step S72 has derived NO, is judged (step S76). When thisjudgment derives route P as the answer, the car-to-car alert alarm isselected (step S78) because the degree of danger is not very high,notwithstanding the distance Lc to other car is so smaller than thesafety car-to-car distance D₁ (Lc<D₁) as to suggest necessity foralerting the driver. In contrast, the judgment derives route Q as theanswer, the car-to-car alert alarm is selected (step S77) because thisjudgment occurs when YES is selected at the step S71, namely when thedistance Lc to the other car is not less than the safety car-to-cardistance D₁ due to the foot transfer time T₁ for car-to-car alarm(Lc≧D₁), and further because the car-to-car alarm is judged on the basisof the foot transfer time T₁.

In short, the question whether a rear-end collision alarm is issued, acar-to-car danger alarm is issued, or a car-to-car alert alarm is issuedis judged in accordance with the first formula of decision (Lc-D₁ ≧0 andLc-D₂ ≧0) and the second formula of decision (S>X(T₂)). Moreover, theuser can freely adjust the timing for issuing the car-to-car alarm andthe rear-end collision alarm separately of each other because the foottransfer time T₁ for car-to-car alarm and the foot transfer time T₂ forrear-end collision alarm can be designated separately of each other bythe user as described above. Accordingly, when the user sets the foottransfer time T₁ for car-to-car alarm at a level shorter than thedriver's own actual speed of reaction with due respect to thepossibility of traffic congestion, for example, the rear-end collisionalarm is infallibly issued in case of genuine need so long as the foottransfer time T₂ for rear-end collision alarm is set at the driver's ownactual speed of reaction (the route of steps S71→S72→S73→S74→S75→S79),notwithstanding the car-to-car danger alarm accompanied by the sound ofalarm is issued only with difficulty (in which case the process willprobably follow chiefly the route of steps S71→S72→S73→S74→S75→S76→S77).

Meanwhile, basically the same process is carried out even when the owncar's deceleration is not less than 10% and yet the setting of thesafety car-to-car distance D has been completed. At the subsequent stepS80, the question whether or not the distance to other car Lc set at thestep S64 is not less than the safety car-to-car distance D found at thestep S70, namely whether or not the following formula (first formula ofdecision) is satisfied, is judged.

    Lc-D≧0

When this judgment derives YES as the answer, the car-to-car alert alarmis selected (step S84). Here, the operation for deciding between theselection of the car-to-car alert alarm of the green color and theselection of the car-to-car alert alarm of the orange color is performedsimilarly in the former case. In contrast, when the judgment derives NOas the answer, the difference of speed S (=V₂ -V₁) between the own carand the other car is computed (step S81), the reference value X(T₂) isfound by reading the foot transfer time T₂ for rear-end collision alarmout of the memory (step S82), and the question whether or not thedifference of speed S is greater than the reference value X (T₂), namelythe question whether or not the following formula (second formula ofdecision) is satisfied, is judged.

    S>X(T.sub.2)

When this judgment derives YES as the answer, the rear-end collisionalarm is selected (step S86). When the judgment derives NO as theanswer, the car-to-car danger alarm is selected (step S85).

FIG. 13-FIG. 15 are flow charts of the processing for judging dangerwith respect to a fixed object shown in FIG. 9.

Basically, these flow charts are entirely the same as those of theprocessing for judging danger with respect to a vehicle shown in FIG.10-FIG. 12. Since it differs only in that the fixed object does not needto consider the speed (the case of the speed of the other car V₁ =0 isconceivable) and that the corrected d value (step S44 in FIG. 8 refers)is used when the fixed object happens to be such as a guardrail, thefurther description will be contracted to the following briefexplanation.

First, the question whether or not the weather is rainy (step S91) isjudged. When this judgment derives YES as the answer, the own car'sbraking distance f₂ (v₂) is corrected, for example, to 1.5 times thestandard value (FIG. 19 refers) (step S92). Then, the distance Ls to thefixed object is set (step S93). The distance Ls in this case has themagnitude of the distance L_(j2) of the fixed object found in the secondmeasurement in the direction j.

Thereafter, the question whether or not the own car's deceleration isnot less than 10% is judged (step S94). When this judgment derives NO asthe answer, for the purpose of drawing the judgment of danger takinginto account the foot transfer time, the foot transfer time T₁ forcar-to-car alarm is read out (step S95), the safety car-to-car distanceD₁ is computed by the following formula (step S96),

    D.sub.1 =f.sub.2 (V.sub.2)+f.sub.3 (T.sub.1)-d

then, the foot transfer time T₂ for rear-end collision alarm is read out(step S97) and the safety car-to-car distance D₂ is computed by thefollowing formula (step S98).

    D.sub.2 =f.sub.2 (V.sub.2)+f.sub.3 (T.sub.2)-d

In contrast, when the judgment derives NO as the answer, the safetycar-to-car distance D is computed by the following formula (step S99)because the deceleration has been already proceeding and the foottransfer time does not need to be taken into account.

    D=f.sub.2 (V.sub.2)-d

As respects to the value of d to be used in the formula given above (thecar-to-car distance involved at the time of safety stop), the valueresulting from correcting the standard value (FIG. 20 refers) (step S44in FIG. 8 refers) is used as already described in order that the alarmis not easily issued when the fixed object happens to be a guardrailetc.

When the setting of the safety car-to-car distances D₁ and D₂ iscompleted where the own car's deceleration is less than 10%, thequestion whether or not the distance Ls to the fixed object is not lessthan the safety car-to-car distance D₁, namely whether or not thefollowing formula (first formula A of decision) is satisfied, is judged(step S100).

    Ls-D.sub.1 ≧0

When this judgment derives NO as the answer (route P), the flow ofprocess immediately advances to the step S102. When the judgment derivesYES as the answer, further, similarly the question whether or not thedistance Ls to the fixed object is not less than the other safetycar-to-car distance D₂, namely the question whether or not the followingformula (first formula B of decision) is satisfied, is judged (stepS101).

    Ls-D.sub.2 ≧0

When this judgment derives YES as the answer, the car-to-car alert alarmis selected (step S105). When the judgment derives NO as the answer, theflow of process advances to the step 102. Incidentally, when thecar-to-car alert alarm is selected, the prescribed operation is furthercarried out to select either the car-to-car alert alarm in the greencolor or the car-to-car alert alarm in the orange color.

At the step S102, the foot transfer time T₂ for rear-end collision alarmis read out to find the reference value X(T₂) (FIG. 21 refers).Thereafter, the question whether or not the own car's speed V₂ isgreater than the reference value X(T₂), namely whether or not thefollowing formula (second formula of decision) is satisfied, is judged(step S103).

    V.sub.2 >X(T.sub.2)

When the judgment derives YES as the answer, the rear-end collisionalarm is selected (step S107). When the judgment derives NO as theanswer, the question whether the route reaching the judgment at the stepS103 is P or Q is judged (step S104). When this judgment derives theroute P, the car-to-car danger alarm is selected (step S106). When thejudgment derives the route Q as the answer, the car-to-car alert alarmis selected (step S105).

Meanwhile, when the setting of the safety car-to-car distance D iscompleted where the own car's deceleration is not less than 10%, firstthe question whether or not the distance Ls to the fixed object is notless than the safety car-to-car distance D, namely whether or not thefollowing formula (first formula of decision) is satisfied (step S108).

    Lc-D≧0

When this judgment derives YES as the answer, the car-to-car alert alarmis selected (step S111). Here again, the prescribed operation is furtherperformed to select either the car-to-car alert alarm in the green coloror the car-to-car alert alarm in the orange color. In contrast, when thejudgment derives NO as the answer, the foot transfer time T₂ forrear-end collision alarm is read out to find the reference value X(T₂)(step S109) and the question whether or not the own car's speed V₂ isgreater than the reference value X(T₂), namely whether or not thefollowing formula (second formula of decision) is satisfied, is judged(step S110).

    S>X(T.sub.2)

When this judgment derives YES as the answer, the rear-end collisionalarm is selected (step S113). When the judgment derives NO as theanswer, the car-to-car danger alarm is selected (step S112).

In this case again, therefore, the question whether a rear-end collisionalarm is issued, a car-to-car danger alarm is issued, or a car-to-caralert alarm is issued is judged by using the first formula of decision(Lc-D₁ ≧0 and Lc-D₂ ≧0 or the like) and the second formula of decision(V₂ >X(T₂) or the like) and, moreover, the foot transfer time T₁ forcar-to-car alarm and the foot transfer time T₂ for rear-end collisionalarm can be designated separately of each other. Accordingly, even whenthe foot transfer time T₁ for car-to-car alarm is set so as to renderthe issuance of the car-to-car danger alarm difficult with due respectto the possibility of traffic congestion, for example, the rear-endcollision alarm is infallibly issued so long as the foot transfer timeT₂ for rear-end collision alarm is set at the actual driver's own speedof reaction.

FIG. 16 is a flow chart of the processing for display of alarm shown inFIG. 5.

In this processing for display of an alarm, first the question what kindof alarm has been selected in the processing for judging danger at thestep S53 or the step S54 is judged (step S121, step S122). When thisjudgment finds the rear-end collision alarm as the answer, the rear-endcollision alarm is displayed, namely the red color LED 22c disposed inthe display part 10 is flickered (step S123). When the judgment findsthe car-to-car danger alarm as the answer, the car-to-car danger alarmis displayed, namely the red color LED 22c is lighted (step S124). Whenthe judgement finds the car-to-car alert alarm as the answer, thecar-to-car alert alarm is displayed, namely the orange color LED 22b orthe green color LED 22a is lighted, depending on the degree of danger(step S125).

When the rear-end collision alarm or the car-to-car danger alarm isselected, the sound of alarm is issued to arouse the driver's attention(step S128) only when the user has set the sound of alarm at the ONstatus (step S126) and he has not applied the brakes (step S127). In anyother case, namely when the user has set the sound of alarm at the OFFstatus or he has already applied the brakes, the sound of alarm is notissued notwithstanding the rear-end collision alarm or the car-to-cardanger alarm has been selected. Absolutely no sound of alarm is issuedin the case of the car-to-car alert alarm as described above.

Then, at the step s129, the distance Lc or Ls to the object formeasurement is displayed in the unit of 1 m, for example, in the ledpart 21 which is provided in the display part 10.

What is claimed is:
 1. An automotive car-to-car distance alarm deviceadapted to measure a distance between a user's own automobile and anobject existing ahead of the own automobile, judge the presence orabsence of danger based on distance data thus obtained, and issue analarm on the case where it is judged to be dangerous, characterized bycomprising:distance measuring means for emitting a light beam in aprescribed plurality of directions and measuring a distance to an objectin the plurality of directions; first discriminating means for comparinga change of the distance measured in the first and the secondmeasurement in the same direction by said distance measuring means witha distance of travel of the own automobile and discriminating whether anobject existing in that direction is a moving object or a fixed object;second discriminating means for, when said first discriminating meanshas judged the presence of not less than a prescribed number of fixedobjects ahead of the own automobile, regressing distance data thereof tofind the standard deviation and comparing the obtained standarddeviation with a prescribed value thereby discriminating whether or notthe fixed objects are exclusively nonobstacles incapable of obstructingthe travel of the own automobile; danger deciding means for deciding thepresence or absence of danger by a prescribed formula of decision,depending on the kind of object discriminated by said firstdiscriminating means or said second discriminating means; and alarmissuing means for issuing a prescribed alarm when said danger decidingmeans has judged the presence of danger.
 2. An automotive car-to-cardistance alarm device according to claim 1, wherein said distancemeasuring means emits the light beam in a prescribed plurality ofdirections as shifted by a prescribed angle.
 3. An automotive car-to-cardistance alarm device according to claim 1, wherein said danger decidingmeans corrects a numerical value of a prescribed parameter forming theformula of decision so as to render the issuance of an alarm difficultwhen the fixed objects are exclusively nonobstacles.
 4. An automotivecar-to-car distance alarm device according to claim 1, wherein saiddanger judging means corrects a numerical value of a prescribedparameter forming the formula of decision in accordance with weather. 5.An automotive car-to-car distance alarm device adapted to measure adistance between a user's own automobile and an object existing ahead ofthe own automobile, judge the presence or absence of danger based ondistance data thus obtained, and issue an alarm on the case where it isjudged to be dangerous, characterized by comprising:distance measuringmeans for emitting a light beam in a prescribed plurality of directionsand measuring a distance to an object in the plurality of directions;first discriminating means for comparing a change of the distancemeasured in the first and the second measurement in the same directionby said distance measuring means with a distance of travel of the own,automobile and discriminating whether an object existing in thatdirection is a moving object or a fixed object; second discriminatingmeans for, when said first discriminating means has judged the presenceof not less than a prescribed number of fixed objects ahead of the ownautomobile, regressing distance data thereof to find the standarddeviation and comparing the obtained standard deviation with aprescribed value thereby discriminating whether or not the fixed objectsare exclusively nonobstacles incapable of obstructing the travel of theown automobile; danger deciding means provided with a first alarm to beissued when it is in such a state as urges alert and necessitatesdeceleration or application of brakes and a second alarm to be issuedwhen it is in such a state as deserves alert and yet falls short ofnecessitating deceleration or application of brakes and adapted toselectively judge the question whether the first alarm is issued, thesecond alarm is issued, or neither of the alarms is issued in accordancewith a first formula of decision formed of one or more formulas ofdecision taking into account a safety car-to-car distance and a secondformula of decision taking into account a difference of speed betweenthe own car and the object, depending on the kind of objectdiscriminated by said first discriminating means or said seconddiscriminating means; and alarm issuing means for issuing an alarm inaccordance with the result of the decision by said danger decidingmeans.
 6. An automotive car-to-car distance alarm device according toclaim 5, wherein each timing for issuing the first alarm and the secondalarm can be adjusted separately of each other by the user.
 7. Anautomotive car-to-car distance alarm device according to claim 6,wherein said adjustment of each timing for issuing the first alarm andthe second alarm is effected by the setting of respective foot transfertimes.
 8. An automotive car-to-car distance alarm device according toclaim 5, wherein the first alarm is displayed by the flickering of a redlamp and the second alarm is displayed at least by the lighting of a redlamp.
 9. An automotive car-to-car distance alarm device according toclaim 5, wherein said distance measuring means emits the light beam insix directions as shifted by a prescribed angle.
 10. An automotivecar-to-car distance alarm device according to claim 5, wherein saiddanger deciding means corrects a numerical value of a prescribedparameter forming the formula of decision so as to render the issuanceof an alarm difficult when the fixed objects are exclusivelynonobstacles.
 11. An automotive car-to-car distance alarm deviceaccording to claim 5, wherein the danger deciding means corrects anumerical value of a prescribed parameter forming the formula ofdecision in accordance with weather.